Microstructure And Corrosion Behavior Of Aluminum Layered Composites

The aluminum brazing sheet material has been widely used in automotive heat exchanger field for its low density,favorable thermal conductivity,satisfactory mechanical properties.The common brazing sheet is composed of an AA3 xxx aluminum alloy clad on both faces with an AA4 xxx aluminum alloy.Controlled atmosphere brazing(CAB)is commonly used in heat exchanger manufacture.This process will change the microstructure,type,distribution and chemical composition of the existing particles as well as the distribution of alloying elements.These particles will lead to localized corrosion problems when subjected to aggressive environments,which could eventually lead to breakdown of the engine due to overheating.The current work aims at investigating a suitable electrochemical technique to elucidate the corrosion propagation behavior of the modified multilayer aluminum brazed sheet by in-situ EN technique combined with ex-situ EIS technique.The cross-sectional depth microstructure of the aluminum layered composites,namely the precipitates free zone(PFZ),the band of dense precipitates(BDP)zone and core material were observed and determined by focused ion beam-transmission electron microscopy(FIB-TEM)and energy dispersive spectrometer(EDS).The theoretical corrosion potential of the BDP zone and core materials was calculated according to the theoretical corrosion potential model.The measured corrosion potentials of different layers of the multilayer aluminum shee were also carried out according to ASTM G69-97 standard test method.The corrosion process of the aluminum layered composites in the typical solution was investigated by electrochemical impedance spectroscopy(EIS)and electrochemical noise(EN)techniques.The equivalent circuit,wavelet transform analysis and cluster analysis methods were employed to research the EIS and EN data.With the immersion time increased,the capacitive time constant in EIS plot changed from one capacitive time constant through two unclear capacitive time constants to two clear capacitive time constants,and then went back to two unclear capacitive time constants again.The results show that there exist four different corrosion stages for the multi-layer modified aluminum brazed sheet in EXCO solution,and the corrosion process can be distinguished by cluster analysis and the energy distribution plot(EDP)extracted from EN data.These electrochemical test results correspond well with the immersion test results.The corrosion propagation process could be identified into four stages: the dissolution of the eutectic ?-Al in the re-solidified clad in form of pitting corrosion;the corrosion of the primary ?-Al grain boundaries in form of inter-granular corrosion(IGC);the corrosion of the BDP zone in form of exfoliation corrosion;the corrosion of the core material in form of IGC.The service life of the multi-layer brazed sheet was tested by the sea water acidic acid test(SWAAT)which commonly used in heat exchanger manufacture in industry.The results show that the multi-layer brazed sheet does not perforate for 25 days in the SWAAT test,compared with other materials exhibits excellent corrosion resistance.